1. Field of the Invention
Organic light emitting diodes (OLEDs) comprise a particularly advantageous form of electro-optic display. They are bright, colorful, fast switching, provide a wide viewing angle and are easy and cheap to fabricate on a variety of substrates.
2. Related Technology
Organic (which here includes organometallic) LEDs may be fabricated using either polymers or small molecules in a range of colors, depending upon the materials used. Examples of polymer-based organic LEDs are described in WO 90/13148, WO 95/06400 and WO 99/48160; examples of small molecule based devices are described in U.S. Pat. No. 4,539,507 and examples of dendrimer-based materials are described in WO 99/21935 and WO 02/067343.
A basic structure 100 of a typical organic LED is shown in FIG. 1a. A glass or plastic substrate 102 supports a transparent anode layer 104 comprising, for example, indium tin oxide (ITO) on which is deposited a hole transport layer 106, an electroluminescent layer 108 and a cathode 110. The electroluminescent layer 108 may comprise, for example, PEDOT: PSS (polystyrene-sulphorate-doped polyethylene-dioxythiophene). Cathode layer 110 typically comprises a low work function metal such as calcium and may include an additional layer immediately adjacent electroluminescent layer 108, such as a layer of aluminum, for improved electron energy level matching. Contact wires 114 and 116 to the anode and the cathode respectively provide a connection to a power source 118. The same basic structure may also be employed for small molecule devices.
In the example shown in FIG. 1a light 120 is emitted through transparent anode 104 and substrate 102 and such devices are referred to as “bottom emitters”. Devices which emit through the cathode may also be constructed, for example, by keeping the thickness of cathode layer 110 less than around 50-100 mm so that the cathode is substantially transparent.
Organic LEDs may be deposited on a substrate in a matrix of pixels to form a single or multi-color pixilated display. A multi-colored display may be constructed using groups of red, green and blue emitting pixels. In such displays the individual elements are generally addressed by activating row (or column) lines to select the pixels, and rows (or columns) of pixels are written to, to create a display. So-called active matrix displays have a memory element, typically a storage capacitor and a transistor, associated with each pixel while passive matrix displays have no such memory element and instead are repetitively scanned, somewhat similarly to a TV picture, to give the impression of a steady image.
FIG. 1b shows a cross-section through a passive matrix OLED display 150 in which like elements to those of FIG. 1a are indicated by like reference numerals. In the passive matrix display 150 the electroluminescent layer 108 comprises a plurality of pixels 152 and the cathode layer 110 comprises a plurality of mutually electrically insulated conductive lines 154, running into the page in FIG. 1b, each with an associated contact 156. Likewise the ITO anode layer 104 also comprises a plurality of anode lines 158, of which only one is shown in FIG. 1b, running at right angles to the cathode lines. Contacts (not shown in FIG. 1b) are also provided for each anode line. An electroluminescent pixel 152 at the intersection of a cathode line and anode line may be addressed by applying a voltage between the relevant anode and cathode lines.
Referring now to FIG. 2a, this shows, conceptually, a driving arrangement for a passive matrix OLED display 150 of the type shown in FIG. 1b. A plurality of constant current generators 200 are provided, each connected to a supply line 202 and to one of a plurality of column lines 204, of which for clarity only one is shown. A plurality of row lines 206 (of which only one is shown) is also provided and each of these may be selectively connected to a ground line 208 by a switched connection 210. As shown, with a positive supply voltage on line 202, column lines 204 comprise anode connections 158 and row lines 206 comprise cathode connections 154, although the connections would be reversed if the power supply line 202 was negative with respect to ground line 208.
As illustrated pixel 212 of the display has power applied to it and is therefore illuminated. To create an image connection 210 for a row is maintained as each of the column lines is activated in turn until the complete row has been addressed, and then the next row is selected and the process repeated. Alternatively a row may be selected and all the columns written in parallel, that is a row selected and a current driven into each of the column lines simultaneously, to simultaneously illuminate each pixel in a row at its desired brightness. Although the latter arrangement requires more column drive circuitry it is preferred because it allows a more rapid refresh of each pixel. In a further alternative arrangement each pixel in a column may be addressed in turn before the next column is addressed, although this is generally not preferred because of the effect, inter alia, of column capacitance as discussed below. It will be appreciated that in the arrangement of FIG. 2a the functions of the column driver circuitry and row driver circuitry may be exchanged.
It is usual to provide a current-controlled rather than a voltage-controlled drive to an OLED because the brightness of an OLED is determined by the current flowing through it, thus determining the number of photons it outputs. In a voltage-controlled configuration the brightness can vary across the area of a display and with time, temperature, and age, making it difficult to predict how bright a pixel will appear when driven by a given voltage. In a color display the accuracy of color representations may also be affected.
FIGS. 2b to 2d illustrate, respectively the current drive 220 applied to a pixel, the voltage 222 across the pixel and the light output 224 from the pixel over time 226 as the pixel is addressed. The row containing the pixel is addressed and at the time indicated by dashed line 228 the current is driven onto the column line for the pixel. The column line (and pixel) has an associated capacitance and thus the voltage gradually rises to a maximum 230. The pixel does not begin to emit light until a point 232 is reached where the voltage across the pixel is greater than the OLED diode voltage drop. Similarly when the drive current is turned off at time 234 the voltage and light output gradually decay as the column capacitance discharges. Where the pixels in a row are all written simultaneously, that is where the columns are driven in parallel, the time interval between times 228 and 234 corresponds to a line scan period.
FIG. 3 shows a schematic diagram 300 of a generic driver circuit for a passive matrix OLED display. The OLED display is indicated by dashed line 302 and comprises a plurality n of row lines 304 each with a corresponding row electrode contact 308 and a plurality n of column lines 308 with a corresponding plurality of column electrode contacts 310. An OLED is connected between each pair of row and column lines with, in the illustrated arrangement, its anode connected to the column line. A y-driver 314 drives the column lines 308 with a constant current and an x-driver 316 drives the row lines 304, selectively connecting the row lines to ground. The y-driver 314 and x-driver 316 are typically both under the control of a processor 318. A power supply 320 provides power to the circuitry and, in particular, to y-driver 314.
FIG. 4 shows schematically the main features of a current driver 402 for one column line of a passive matrix OLED display, such as the display 302 of FIG. 3. Typically a plurality of such current drivers are provided in a column driver integrated circuit, such as y-driver 314 of FIG. 3, for driving a plurality of passive matrix display column electrodes.
The current driver 402 of FIG. 4 outlines the main features of this circuit and comprises a current driver block 406 incorporating a bipolar transistor 416 which has an emitter terminal substantially directly connected to a power supply line 404 at supply voltage Vs. (This does not necessarily require that the emitter terminal should be connected to a power supply line or terminal for the driver by the most direct route but rather that there should preferably be no intervening components, apart from the intrinsic resistance of tracks or connections within the driver circuitry between the emitter and a power supply rail). A column drive output 408 provides a current drive to OLED 412, which also has a ground connection 414, normally via a row driver MOS switch (not shown in FIG. 4). A current control input 410 is provided to current driver block 406 and, for the purpose of illustration, this is shown connected to the base of transistor 416 although in practice a current mirror arrangement is preferred. The signal on current control line 410 may comprise either a voltage or a current signal. Where the current driver block 406 provides a variable controllable current source each current driver block may be interfaced with and controlled by an analog output from a digital to analog converter. Such a controllable current source can provide a variable brightness or grayscale display. Other methods of varying pixel brightness include varying pixel on time using Pulse Width Modulation (PWM). In a PWM scheme a pixel is either fully on or completely off but the apparent brightness of a pixel varies because of time integration within the observer's eye.